CMH vs QB in a cooler environment: advice requested

Hi all!

I'm setting up for a grow in a relatively cool basement, around 55-65 degrees F, in a 2x4 tent.

I had been planning on buying the HLG 260 QB kit, but I'm concerned about maintaining enough heat in a tent. As an alternative, I was considering a 315 CMH set-up, which I assume would generate more heat, probably to my benefit in the cooler environment.

I don't want to mess around with supplemental heat, for cost and safety concern reasons, if possible.

My question: In a cooler 55-65 degree environment, which light would maintain the right temperature environment in a 2x4 tent, the QB or the CMH?

Not sure about heat. But I would want to consider tent height / light distances.

Thanks! I agree, I'm aiming for a 6 ft tall tent if I go with a CMH. But, if a smaller sq.ft. (5 ft tent) and QB would heat the tent enough, that could work. I basically need to increase the ambient environment temp by 15-20 degrees inside the tent. I'd prefer the QB for efficiency and size, but if the tent would be way too cool for recommended growing temps, then it is a deal-breaker in my current circumstance. The CMH have other advantages such as UV, and in my case I guess it might be helpful to run hotter and get the tent temps up enough.

I'd really appreciate help from folks with experience with this specific issue. If anyone grows in a small tent in a cool environment, what type of light gets your tent to a nice temperature?

I would just use an HID light, CMH, HPS...

With LED you actually need warmer temps than you do with HID. You also need to ventilate the tent some to keep CO2 levels up unless you are gonna run a bottle. The ventilation will cool the tent some.

Thanks so much for the advice!

I'm planning on a very mild ventilation, due to the cool environment. But I really want to scrub any smell due to my wife's parents frequent visits. So I'm thinking I have to have a carbon filter and a small 4" fan (likely AC Infinity for low sound and efficiency), maybe even at a reduced speed.

So, hopefully, I can squeeze a CMH into a 2x4, vent gently, and still maintain a solid 15 degree temp increase. I'm not convinced that even the CMH will heat the tent enough, but I'm hoping to hear from someone that this plan could work!

Well if you want to ventilate really lightly, just enough to hold CO2 levels, you can put a speed control on the exhaust fan, but for odor control you do need just enough to keep a negative pressure. A variac is the best speed controller, some speed controllers make the motor groan at reduced speeds. Also the slowed blower will be quieter so that can help with stealth. Still I think with such low ambient temps the tent will stay cool, like in the low to mid 70's. Since you need warmer temps with LED compared to HID you may find yourself looking to add heat. Also when the lights go off the tent will get cold and that will limit phosphorous uptake and can reduce growth to a crawl.

Also the colder night temps can cause the RH% to spike and that can cause some major issues like PM. So you may be forced to keep some ventilation during lights off to prevent humidity from rising. This compounds the problem with cold roots.

Thanks Renfro!! I had not considered the temp drops at night! I may have to reconsider my location. I don't know if I want to mess with heating the basement.

What temperature would be acceptable during the "night" phase when lights are off? The temp can drop somewhat and still be healthy, right?

BTW I will be growing mostly indica strains, which I did read were a little bit more cold resistant than Sativas, if that helps.

Ideally you would keep the lights off temperature drop to no more than 10 degrees F. Definitely try to avoid going below 65f.

Being cold resistant is good for handling temporary conditions but if you have cool temps the whole grow your plants will grow much slower and yields will suffer. Also try to avoid large temperature and humidity swings as this can cause issues like powdery mildew.

Would it be possible to run some duct to the upstairs to access warmer air for the tent intake? If you have central air you might find it simple to tap into the ductwork and grab some warm air from that. Just spitballing here... They make those electric dampers for round duct, one could use one on a thermostat to open the warm air flow when temps are below a set point.

With either setup, I would use a dehumidifier to help with humidity and add heat when needed. And yes, you can use the air from inside, which will have a higher c02 level.

I also like the hyperfans because of the excellent efficiency and the dimmer. Hook this up to a thermostat/humidity controller.

We do have central air, that's an interesting idea. TBH, my mind has now shifted to moving the grow inside the house. Might have to concede some size and keep things as stealthy as possible, but the temp in the house is perfect and controlled, wouldn't swing a lot, and has good humidity. My wife understandably wanted it outside or in the basement, but she'll understand the logic of the basement being a poor environment, she's cool haha.

Thank you all so much for your help!!

buckyshimp said:

Thanks Renfro!! I had not considered the temp drops at night! I may have to reconsider my location. I don't know if I want to mess with heating the basement.

What temperature would be acceptable during the "night" phase when lights are off? The temp can drop somewhat and still be healthy, right?

BTW I will be growing mostly indica strains, which I did read were a little bit more cold resistant than Sativas, if that helps.

Click to expand...

Any temperature drop that keeps your humidity from spiking is good.

Frankly, no temperature drop at all is best.

CAn you fit a 3x3 tent instead if you haven’t already got it I recommend that with 315 and with the ac infinity fan your day time temps will be fine. Your night temps are good if it’s 65 if it’s 55 you will need supplemental heat when lights are off. Or you could run autoflowers if your not against that and then your light will heat the whole time. If it’s only 55 in winter just run autos in winter. Those oil filled radiator heaters are fairly safe compared to most space heaters and in that small of a space it’s not gonna take much power to heat it if that’s option you decide to go with. If you got a basement utilize it most of us without them wish we had them. Plants smell sometimes even with a filter they will be way easier to conceal in a basement where most visitors should never be I would think.

if your basement isn’t just wide open and you have seperate rooms then you can use a room as a lung room and run two tents on opposite schedules in there. Just set an ac infinity fan to vent the whole room if it’s over 65 degrees. My 315’s add about 10 degrees To the tent over ambient temps. So if you keep the room at 65 your temps with light on should be about 75 and then when the light kicks off the ambient temps of 65 is perfect. You can also use your lung room for your dehumidifier if you need one as they are kinda hard to fit in a small tent.

Edit: I vent my tents with one fan and filter sitting in between them it goes like this filter is on the floor 6 inch fan on top blowing down thru the filter. Afther the fan is a y on top of that I then put a 4 to to 6 inch reducer on top of the y and run 4 inch duct to both tents. Both tents are 3x3 with a 315 in each.

And honestly if I could only grow in a place with nite temps of 55 I’d still do it some strains won’t like it and it’s not ideal but I honestly think for personal smoke the only thing it would hurt would be a little bit of yield. Keep something between the pots and the floor to insulate them and you will be fine.

ttystikk said:

Any temperature drop that keeps your humidity from spiking is good.

Frankly, no temperature drop at all is best.

Click to expand...

Have to say its kinda dependent on the growth type you are getting and the growth type you want. Running LED with with no temp diff and an indica dominant stain is not likely going to be ideal. You will be growing leaves on top of leaves with virtually no node spacing.

I think ideal day/night temp diffs will vary based on lighting, strain and grow style

I’d get the CHM. Rum it at night when it’s cooler outside to offset the temperature.. then go from there.. maybe a small space heater if things are dropping to low in daytime.. jm2c